Early work relating to network-based Wireless Location Systems (WLSs) is described in U.S. Pat. No. 4,728,959; “Direction Finding Localization System” (issued Mar. 1, 1998) which discloses a system for locating cellular telephones using angle of arrival (AOA) techniques and U.S. Pat. No. 5,327,144, (Issued Jul. 5, 1994) “Cellular Telephone Location System,” which discloses a system for locating cellular telephones using time difference of arrival (TDOA) techniques. Further enhancements of the system disclosed in the '144 patent are disclosed in U.S. Pat. No. 5,608,410, (Issued Mar. 4, 1997), “System for Locating a Source of Bursty Transmissions.” Location estimation techniques for wide-band wireless communications systems were further developed in U.S. Pat. No. 6,047,192 (Issued April 4, 200), “Robust, Efficient Localization System.”
All of these patents are assigned to TruePosition, Inc., the assignee of the present invention. TruePosition has continued to develop significant enhancements to the original inventive concepts. First commercially deployed in 1998 by TruePosition in Houston, Tex., overlay network-based wireless location systems have been widely deployed in support of location-based services including emergency services location. The ability to routinely, reliably, and rapidly locate cellular wireless communications devices has the potential to provide significant public benefit in public safety and convenience and in commercial productivity.
Use of constraints on signal time and frequency deviations from an expected value has previously been considered in TruePosition's U.S. Pat. No. 6,876,859; “Method for estimating TDOA and FDOA in a wireless location system.” This patent taught an approach that limited the initial search space based on the known distance of receivers to the wireless location system. The frequency search range was similarly constrained to a range accommodating the likely velocity of the mobile device.
The inventive techniques and concepts described herein apply to time and frequency division multiplexed (TDMA/FDMA) radio communications systems including the widely used IS-136 (TDMA), GSM, and OFDM (e.g. eUTRAN/LTE, IEEE 802.16 and IEEE 802.20) wireless systems, as well as code-division radio communications systems such as CDMA (IS-95, IS-2000) and Universal Mobile Telecommunications System (UMTS), the latter of which is also known as W-CDMA. There are many more types of air interfaces used throughout the world, and there is no intent to exclude any air interface from the inventive concepts described within this specification. Indeed, those skilled in the art will recognize other radio air interfaces used elsewhere are derivatives of or similar in class to those described above.
A position estimate typically requires multiple baseline measurements that can be corrupted by factors such as noise and fading. Errors that are caused by these factors may be accounted for by weighing the contribution of measurements in the position estimation algorithm based on knowledge of the type of degradation. However, one or more relatively poor measurements may badly skew an otherwise low error position estimate.